The present invention relates generally to mass transfer devices for use with chemical process and mass transfer reactors and columns and, more particularly, to random packing elements for use in such reactors and columns. The invention also relates to methods of making and using such random packing elements.
Random or dumped packing elements are normally employed in gas-liquid or liquid-liquid contact towers or columns to provide mass transfer surfaces between a downwardly flowing fluid, typically a liquid stream, and an upwardly ascending fluid, typically gas or vapor stream or another liquid stream. Random packing elements may be used in a variety of chemical and treatment processes, such as, for example, rectification, stripping, fractionating, absorbing, separating, washing, extraction, or any other chemical, heat exchange, or treatment-type processes. Generally, the discrete random packing elements have a specific geometric shape and are designed to maximize performance for a given mass transfer surface area. Because the random packing elements are generally dumped or randomly packed into the column shell in an arbitrarily orientated packed bed, it is desirable for the individual random packing elements to have both high mass transfer efficiency and good hydraulic capacity when positioned in multiple rotational orientations within the packed bed.
Random packing elements of the prior art exist in a variety of shapes and materials. In general, random packing elements are constructed of metal, ceramic-type material, plastics, glass, or the like. Commonly, random packing elements are cylindrical, arcuate or “saddle-shaped” or have other, non-arcuate shapes such as spherical, toroidal, and the like. One disadvantage of the random packing elements of the prior art is that often the performance of the element is highly dependent on its configuration and its orientation with respect to the direction of flow of fluid streams through the element within the packed bed. For example, a Pall ring is a well-known cylinder-type packing that has a plurality of slotted walls and internal tongues or projections. When viewed along its longitudinal axis, the Pall ring presents very little surface area for mass transfer, but, when viewed perpendicularly to its longitudinal axis, the element presents a very large surface area. Because of this difference, the surface areas available for vapor/liquid or liquid/liquid contact vary with the orientation of the element, which, ultimately, affects the element's performance. In addition, the large surface area in the direction perpendicular to the longitudinal axis of the Pall ring is disadvantageous in that it tends to “shield” or inhibit fluid flow through immediately adjacent rings in the downstream flow direction.
A random packing element having a more uniform distribution of surface area throughout its entire volume is disclosed in U.S. Pat. No. 5,112,536 (also referred to herein as the '536 patent). The saddle-shaped random packing element disclosed therein includes alternating arched inner and outer rib elements that define an interior volume. While the packing element disclosed in the '536 patent provides improvements over other random packing elements of the prior art, it also includes limitations that can hinder its performance. For example, the curvature of the packing element disclosed in the '536 patent causes the inner rib elements to be positioned closely together in the same curved plane, thereby restricting the flow paths for fluid streams passing through the element. In addition, because the ribs of the packing element of the '536 patent are generally centrally longitudinally aligned, the first rib element encountered by a fluid stream typically shields the subsequent rib elements from fluid contact. This shielding effect can reduce mass transfer efficiency by reducing the element's effective surface area for mass and/or heat transfer.
Another type of random packing element is disclosed in U.S. Pat. No. 5,882,772. In that patent, a plurality of individual strips is provided and each strip extends in a generally sinusoidal fashion between planar outer web attachment areas. A similar random packing element is disclosed in U.S. Pat. No. 5,543,088 in which a plurality of strips extends between planar end attachment areas. In both of these patents, the individual strips are connected together along their midpoints, thereby providing an area where the strips are crowded together and through which passage of gas is impeded. In addition, the packing elements must be made from material of sufficient thickness and strength to resist deformation of the planar outer webs or attachment areas. It would be desirable for the random packing element to have a configuration that resists deformation using relatively thinner gauge material.
Thus, a need exists for a random packing element that maintains high mass transfer efficiency and good hydraulic capacity when positioned in multiple different rotational orientations within the packed bed. Advantageously, the packing element should be easily manufactured with little or no waste material and possess a configuration that more readily resists the type of deformation described above.